1. Field of the Invention
The invention relates to a fuel injection system for an internal combustion engine, and more particularly to such a system including a pressure booster.
2. Brief Description of the Prior Art
For better comprehension of the specification and claims, several terms will now be explained: The fuel injection system of the invention can be embodied as either stroke-controlled or pressure-controlled. As used herein, the term, xe2x80x9cstroke-controlled fuel injection systemxe2x80x9d is understood to mean that the opening and closing of the injection opening is effected with the aid of a displaceable valve member on the basis of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. A pressure drop inside the control chamber causes a stroke of the valve member. Alternatively, the deflection of the valve member can be done by a final control element (actuator). In a xe2x80x9cpressure-controlled fuel injection systemxe2x80x9d according to the invention, the valve member is moved counter to the action of a closing force (spring) by the fuel pressure prevailing in the nozzle chamber of an injector, so that the injection opening is uncovered for an injection of the fuel out of the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder is called the xe2x80x9cinjection pressurexe2x80x9d, while the term xe2x80x9csystem pressurexe2x80x9d is understood to mean the pressure at which the fuel is available or kept on hand inside the fuel injection system. xe2x80x9cFuel meteringxe2x80x9d means delivering fuel to the nozzle chamber by means of a metering valve. In xe2x80x9ccombined fuelxe2x80x9d metering, a common valve is used to measure various injection pressures. In the xe2x80x9cunit fuel injectorxe2x80x9d (PDE), the injection pump and the injector form a unit. One such unit per cylinder is built into the cylinder head and driven by the engine camshaft, either directly via a tappet or indirectly via tilting levers. The xe2x80x9cpump-line-nozzle systemxe2x80x9d (PLD) operates by the same method. A high-pressure line in that case leads to the nozzle chamber or nozzle holder.
A unit fuel injector is known from German Patent Disclosure DE 195 175 78 A1. In this fuel injection system, the system pressure is generated via a pressure-actuatable piston, whose motion is controlled by a cam drive. A variable fuel injection of different quantities for the pre-injection, main injection and post-injection can be accomplished only to a limited extent with such a fuel injection system.
The fuel injection system of the present invention makes it possible to achieve the fuel injection with the aid of a unit fuel injector over a wide rpm range with high precision. This makes it possible to remove pollutant exchange and enables more flexible pre-injection and post-injection by means of a unit fuel injector or a pump-line-nozzle system. The teaching according to the invention combines the advantages of a pressure-boosted (pressure-amplified) injector with a non-pressure-boosted unit fuel injector. If a piezoelectric actuator is used for fuel metering, improved metering of the injected fuel quantity can be attained. This creates a good minimum quantity capability in the pre-injection and if needed in the post-injection, because such an injection is done under stroke control with a lesser injection pressure. The pre-injection and post-injection are done flexibly and replicably. Good hydraulic efficiency of the pressure boosting is achieved if the pressure booster is disposed inside the injector. The course of injection can be varied in a targeted way.